Present dietetic needs, predilections, and perceptions have led to the increased use of artificial sweeteners as a replacement for the "natural" sugars, including sucrose and fructose. Such artificial sweeteners are highly imperfect, including being under continual review for their long term physiological affects, yet their demand has grown unabated. Accompanying their growth as a commercial area with substantial economic impact has been a renewed emphasis on discovering and supplying new artificial sweeteners.
The ideal artificial sweetener would be noncaloric, noncariogenic, without detrimental physiological effects, and usable by diabetics. All these requirements would be met if a sweetener were not metabolized by humans and by flora which are found in the mouth and intestinal tract, and if the sweetener were either not absorbed by humans, or absorbed without effect on any internal organ. That is, the ideal sweetener should be excreted in the same form as when ingested. Another desirable feature is that it have bulk properties similar to sucrose so that it can be substituted for table sugar in many formulations. Recently, and perhaps belatedly, attention has turned toward the L-sugars as desirable artificial sweeteners. It has been known since at least 1946 that L-fructose is sweet (M. L. Wolfrom and A. Thompson, J. Am. Chem. Soc., 68, 791,793 (1946)), and since at least 1890 that L-fructose is nonfermentable (E. Fischer, Ber. Deutsch. Chem. Ges., 23, 370,389 (1890)), hence not metabolized by microorganisms generally metabolizing D-sugars. Given that L-fructose is a sweet nonmetabolite it becomes obvious to use it as a noncaloric sweetener in many formulations.
Exploitation of the favorable properties of L-sugars is hindered by their relative unavailability. L-fructose, for example, is not found to any significant extent in nature. This unavailability has spurred recent efforts in developing commercially feasible methods for preparing L-sugars in amounts necessary for their use as a staple of commerce. U.S. Pat. Nos. 4,371,616 and 4,421,568 describe a method of producing L-sugars, including L-idose and L-glucose, from the readily available D-glucose. Although the preparation of a number of L-sugars is described in U.S. Pat. No. 4,262,032 the focus seems to be on typical laboratory methods wholly unsuited for economical industrial production, in contrast to the process herein. U.S. Pat. No. 4,440,855 uses glucose oxidase to convert L-glucose to L-glucosone. To the extent that there are suitable procedures adaptable to the large scale production of L-glucose as well as the conversion of L-glucosone to L-fructose, the teachings of the patentee relate to the preparation of L-fructose. The subject matter of U.S. Pat. No. 4,207,413 is L-sucrose, the enantiomer of ordinary table sugar, which can be hydrolyzed to afford L-fructose.
None of the art cited above fulfills the need for an economically and commercially feasible process for the preparation of L-fructose. Such a process requires, inter alia, a readily accessible starting material, preferably one with the L-configuration. L-arabinose, obtained from the hydrolysis of lignocellulosics, is one of few naturally occurring L-sugars and is abundant. The subject matter of this application is the conversion of L-arabinose to L-fructose. Since the products of this invention include L-glucose and L-mannose as well, it is clear that the process which is this invention relates to their preparation as well as that of L-fructose. Because L-fructose currently is the L-sugar of greatest interest the following material speaks primarily to its preparation, nonetheless it is to be understood that the processes described and claimed are directed toward L-glucose and L-mannose equally well.